The invention relates to a measuring device and method for measuring the oxygen partial pressure in high-temperature, corrosive liquids, in particular, glass and salt melts.
Measuring devices of this kind are known, for example, from DE-PS 31 09 454. Platinum electrodes are generally used as both measuring and reference electrodes. As oxygen-ion conducting solid electrolyte, there is used an oxidic material, generally zirconium dioxide (ZrO.sub.2), provided, if necessary, with suitable doping, for example, doped with a second oxide, usually yttrium oxide (Y.sub.2 O.sub.3). The doping material increases the ion conductivity of the solid electrolyte basic material. The zirconium dioxide doped in this way exhibits an oxygen-ion transport number of 1, which means that the only ions mobile in the electrolyte are oxygen ions.
To obtain a good three-phase contact in the reference electrode arrangement, advantageously, an amount of powdered or granular loose solid electrolyte is brought into the reference space. The platinum reference electrode and the outlet of a reference gas feed pipe extend into this powdered solid electrolyte. At the high temperatures present with glass melts, the solid electrolyte powder sinters during first use so that a gas-permeable, sintered compact is formed that is very suitable as a three-phase contact zone. On this aspect, see also DE-PS 30 28 270. The term "three-phase" contact zone refers to the mutual contact of three elements, i.e., phases. These elements are the platinum electrode wire, the sintered solid electrolyte material, and the reference gas (oxygen).
For external contact of the solid electrolyte with the liquid or melt, there is used a pipe made from oxygen ionizing solid electrolyte, which is closed at the lower end. This pipe, which contains the reference space, is immersed overall in the liquid or melt. The sintered compact of the oxygen-ion conducting electrolyte is placed in the lower, closed end of the pipe and is in direct contact with the wall material of the pipe. In this way, a direct conduction of oxygen ions through the wall of the lower pipe end into the powdered material or sintered compact, and thus into the three-phase contact area, is guaranteed. The other advantage of directly immersing this arrangement into the liquid is that the temperatures at the measuring electrode and at the reference electrode arrangement, which are both immersed into the liquid, can be considered to be approximately the same. A temperature compensation of the measuring voltage obtained with the measuring arrangement is generally not required in this case.
The general functioning of the prior art devices as described herein has been explained by the inventor in his publication, "Development of Electrochemical Cells Employing Oxide Ceramics for Measuring Oxygen Partial Pressures in Laboratory and Technical Glass Melts" [Glastechn. Ber. 56K (1983) Bd. 1].
But a drawback of this arrangement is that, in particular with corrosive, lead-containing glass melts, the pipe made of the solid electrolyte material in the flowing melt is so extensively corroded that it possesses a service life of, at most, one week. Therefore, due to the penetration into glass melt, expensive reference electrodes are useless and have to be rejected.
This drawback is overcome, for example, by using a special contact element in the form of an elongated rod made of oxygen-ion conducting material which is positioned with its upper end in ion-conducting contact with the solid electrolyte in the reference space of the reference electrode arrangement. Only the lower end of the contact element is immersed in the melt. To the extent that corrosion wear of the immersed end of the oxygen-ion conducting rod occurs, the position of the whole reference electrode arrangement can be reset so as to maintain immersion in the melt until the rod is mostly used up. In this way, not only are considerably increased service lives achieved; but, with a suitable embodiment, the spent rod can also be replaced by a new one so that the same reference electrode arrangement can continue to be used. This embodiment can be manufactured less expensively because the outer pipe enclosing the reference space need no longer be made of expensive solid electrolyte material. Instead, the outer pipe can be made, for example, of a material which is neutral to oxygen ion conduction and which is heat resistant, such as aluminum oxide. Reference electrode embodiments of this kind are described in DE-PS 31 09 454.
An embodiment of a reference electrode arrangement, as described in DE-GM 85 13 976, where a rodlike contact element of oxygen-ion conducting material is provided with only its lower end immersed in the melt and exposed to corrosion by the latter, has proven to be effective. With this embodiment, the reference space is in a pipe, made of a highly heat-resistant material and which is open at its lower end, into whose lower end the rodlike contact element is inserted a short distance and held, for example, with a crossbolt.
This embodiment offers in itself the advantage that the contact element, after extensive use, can be exchanged for a new one so that the rest of the relatively expensive reference electrode arrangement can be reused.
But here a problem is presented regarding sufficient sealing of the contact element inside the outer pipe of the measuring arrangement into which it is inserted. Namely, there is the danger that gases and vapors from the atmosphere above the liquid or melt will invade the three-phase contact zone through a remaining gap between the contact element and outer pipe and thereby change the composition of the reference gas, leading to faulty measurements. Attempts have been made to correct this problem by increasing the pressure of the reference gas inside the measuring arrangement so that the reference gas is discharged through the leaks, rather than gases penetrating into the measuring arrangement from the outside. But this solution has proven to be unreliable.